Heat sink with cooling channel

ABSTRACT

A heat sink apparatus is provided with a cooling channel which directs cooling air between groups of fins on either side of the cooling channel to a location adjacent that of an electronic device to be cooled. The increase in cooling efficiency achieved by providing cooling air to the critical location of the electronic device more than offsets any loss in cooling efficiency due to the elimination of fins required for creation of the cooling channel. Thus a heat sink apparatus can be designed providing increased cooling to selected locations without the use of heat pipes or the like.

TECHNICAL FIELD

[0001] The present invention relates to a heat sink capable of managingthe heat from a heat source such as an electronic device.

BACKGROUND OF THE INVENTION

[0002] With the development of more and more sophisticated electronicdevices, including those capable of increasing processing speeds andhigher frequencies, having smaller size and more complicated powerrequirements, and exhibiting other technological advances, such asmicroprocessors and integrated circuits in electronic and electricalcomponents and systems as well as in other devices such as high poweroptical devices, relatively extreme temperatures can be generated.However, microprocessors, integrated circuits and other sophisticatedelectronic components typically operate efficiently only under a certainrange of threshold temperatures. The excessive heat generated duringoperation of these components can not only harm their own performance,but can also degrade the performance and reliability of the overallsystem and can even cause system failure. The increasingly wide range ofenvironmental conditions, including temperature extremes, in whichelectronic systems are expected to operate, exacerbates the negativeeffects of excessive heat.

[0003] With the increased need for heat dissipation from microelectronicdevices, thermal management becomes an increasingly important element ofthe design of electronic products. Both performance reliability and lifeexpectancy of electronic equipment are inversely related to thecomponent temperature of the equipment. For instance, a reduction in theoperating temperature of a device such as a typical siliconsemiconductor can correspond to an increase in the processing speed,reliability and life expectancy of the device. Therefore, to maximizethe life-span and reliability of a component, controlling the deviceoperating temperature within the limits set by the designers is ofparamount importance.

[0004] Several types of heat dissipating components are utilized tofacilitate heat dissipation from electronic devices. The presentinvention is directly applicable to finned heat sinks.

[0005] These heat sinks facilitate heat dissipation from the surface ofa heat source, such as a heat-generating electronic device, to a coolerenvironment, usually air. The heat sink seeks to increase the heattransfer efficiency between the electronic device and the ambient airprimarily by increasing the surface area that is in direct contact withthe air or other heat transfer media. This allows more heat to bedissipated and thus lowers the electronic device operating temperature.The primary purpose of a heat dissipating component is to help maintainthe device temperature below the maximum allowable temperature specifiedby its designer/manufacturer.

[0006] Typically, the heat sinks are formed of a metal, especiallycopper or aluminum, due to the ability of metals like copper to readilyabsorb heat and transfer it about its entire structure. Copper heatsinks are often formed with fins or other structures to increase thesurface area of the heat sink, with air being forced across or throughthe fins (such as by a fan) to effect heat dissipation from theelectronic component, through the copper heat sink and then to the air.

[0007] The use of copper or aluminum heat dissipating elements canpresent a problem because of the weight of the metal, particularly whenthe heat transmitting area of the heat dissipating component issignificantly larger than that of the electronic device. For instance,pure copper weighs 8.96 grams per cubic centimeter (g/cm³) and purealuminum weighs 2.70 g/cm³.

[0008] For example, in many applications, several heat sinks need to bearrayed on, e.g., a circuit board to dissipate heat from a variety ofcomponents on the board. If metallic heat sinks are employed, the sheerweight of the metal on the board can increase the chances of the boardcracking or of other equally undesirable effects, and increases theweight of the component itself. For portable electronic devices, anymethod to reduce weight while maintaining heat dissipationcharacteristics is especially desirable.

[0009] Another group of materials suitable for use in heat sinks arethose materials generally known as graphites, but in particulargraphites such as those based on natural graphites and flexible graphiteas described below. These materials are anisotropic and allow the heatsink to be designed to preferentially transfer heat in selecteddirections. Also, the graphite materials are much lighter in weight andthus provide many advantages over copper or aluminum.

[0010] Graphites are made up of layer planes of hexagonal arrays ornetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another. Thesubstantially flat, parallel equidistant sheets or layers of carbonatoms, usually referred to as graphene layers or basal planes, arelinked or bonded together and groups thereof are arranged incrystallites. Highly ordered graphites consist of crystallites ofconsiderable size: the crystallites being highly aligned or orientedwith respect to each other and having well ordered carbon layers. Inother words, highly ordered graphites have a high degree of preferredcrystallite orientation. It should be noted that graphites possessanisotropic structures and thus exhibit or possess many properties thatare highly directional e.g. thermal and electrical conductivity andfluid diffusion.

[0011] Briefly, graphites may be characterized as laminated structuresof carbon, that is, structures consisting of superposed layers orlaminae of carbon atoms joined together by weak van der Waals forces. Inconsidering the graphite structure, two axes or directions are usuallynoted, to wit, the “c” axis or direction and the “a” axes or directions.For simplicity, the “c” axis or direction may be considered as thedirection perpendicular to the carbon layers. The “a” axes or directionsmay be considered as the directions parallel to the carbon layers or thedirections perpendicular to the “c” direction. The graphites suitablefor manufacturing flexible graphite sheets possess a very high degree oforientation.

[0012] As noted above, the bonding forces holding the parallel layers ofcarbon atoms together are only weak van der Waals forces. Naturalgraphites can be treated so that the spacing between the superposedcarbon layers or laminae can be appreciably opened up so as to provide amarked expansion in the direction perpendicular to the layers, that is,in the “c” direction, and thus form an expanded or intumesced graphitestructure in which the laminar character of the carbon layers issubstantially retained.

[0013] Graphite flake which has been greatly expanded and moreparticularly expanded so as to have a final thickness or “c” directiondimension which is as much as about 80 or more times the original “c”direction dimension can be formed without the use of a binder intocohesive or integrated sheets of expanded graphite, e.g. webs, papers,strips, tapes, foils, mats or the like (typically referred to as“flexible graphite”). The formation of graphite particles which havebeen expanded to have a final thickness or “c” dimension which is asmuch as about 80 times or more the original “c” direction dimension intointegrated flexible sheets by compression, without the use of anybinding material, is believed to be possible due to the mechanicalinterlocking, or cohesion, which is achieved between the voluminouslyexpanded graphite particles.

[0014] In addition to flexibility, the sheet material, as noted above,has also been found to possess a high degree of anisotropy with respectto thermal and electrical conductivity and fluid diffusion, comparableto the natural graphite starting material due to orientation of theexpanded graphite particles and graphite layers substantially parallelto the opposed faces of the sheet resulting from very high compression,e.g. roll pressing. Sheet material thus produced has excellentflexibility, good strength and a very high degree of orientation.

[0015] Briefly, the process of producing flexible, binderlessanisotropic graphite sheet material, e.g. web, paper, strip, tape, foil,mat, or the like, comprises compressing or compacting under apredetermined load and in the absence of a binder, expanded graphiteparticles which have a “c” direction dimension which is as much as about80 or more times that of the original particles so as to form asubstantially flat, flexible, integrated graphite sheet. The expandedgraphite particles that generally are worm-like or vermiform inappearance, once compressed, will maintain the compression set andalignment with the opposed major surfaces of the sheet. The density andthickness of the sheet material can be varied by controlling the degreeof compression. The density of the sheet material can be within therange of from about 0.04 g/cm³ to about 2.0 g/cm³. The flexible graphitesheet material exhibits an appreciable degree of anisotropy due to thealignment of graphite particles parallel to the major opposed, parallelsurfaces of the sheet, with the degree of anisotropy increasing uponroll pressing of the sheet material to increase orientation. In rollpressed anisotropic sheet material, the thickness, i.e. the directionperpendicular to the opposed, parallel sheet surfaces comprises the “c”direction and the directions ranging along the length and width, i.e.along or parallel to the opposed, major surfaces comprises the “a”directions and the thermal, electrical and fluid diffusion properties ofthe sheet are very different, by orders of magnitude, for the “c” and“a” directions.

[0016] With heat sinks of any of these materials, special situations aresometimes encountered where additional cooling is necessary. Suchproblems are currently solved through the use of heat pipes. However,using a heat pipe adds cost, requires extensive machining, and becauseof reliability concerns a second heat pipe is often added to provideredundancy in the event of a failure.

[0017] What is needed is a way to increase cooling at selected locationson a heat sink without the cost and complication of adding heat pipes.

SUMMARY OF THE INVENTION

[0018] The present invention provides a heat sink design which inselected cases can provide improved cooling at selected locations on theheat sink, without the use of heat pipes. Take, for example, the case ofan elongated heat sink having the electronic device which is to becooled placed at a location a relatively long distance away from the endof the heat sink which receives cooling air from a fan. In this case theair which passes over the location of the electronic device is heatedprior to reaching that location because it must pass over an extensivelength of heated cooling fins. Thus, when the air reaches the locationon the heat sink immediately above the device being cooled, the air willhave already been heated by passing over the upstream portions of theheat sink, so that insufficient cooling occurs at the critical location.

[0019] The present invention provides a solution to this problem, byeliminating a portion of the fins which would normally be present on theheat sink thus defining a cooling channel which allows cooling air toflow directly from the fan to the location adjacent the electronicdevice being cooled. This solution is counterintuitive to typical heatsink design, in that it would normally be expected that a reduction inthe surface area of the heat sink by removing fins would cause the heatsink to perform less well. It has been discovered, however, that incertain instances the reduction in cooling efficiency caused byelimination of fins is more than offset by the increase in coolingefficiency at the critical location on the heat sink, due to the coolerair.

[0020] Accordingly, a heat sink apparatus is provided which includes abase having first and second ends, a plurality of fins extending upwardfrom the base, and a cooling channel defined between first and secondgroups of the fins. The cooling channel extends from the first endtoward a location for an electronic device. The cooling channel has achannel width greater than the spacing between adjacent fins within eachof the first and second groups. The apparatus is preferably used incombination with a cooling fan oriented to direct cooling air across theheat sink from the first end toward the second end of the base, and anelectronic device in heat transfer communication with the location onthe base, so that heat from the electronic device is transferred by theheat sink from the electronic device to the cooling air.

[0021] In yet another embodiment of the invention a heat sink apparatusincludes a base having a length and a width, the length being at leastthree times the width. The apparatus includes a plurality of parallelfins extending from the base, the fins including first and second groupsseparated by a cooling channel terminating short of a location for anelectronic device.

[0022] In another embodiment of the invention a method is provided forcooling an electronic device. The method includes providing a heat sinkhaving first and second groups of fins and having a cooling channeldefined between the first and second groups. An electronic device isplaced in heat transfer communication with a location on the heat sink.Cooling air is channeled through the cooling channel to the location ofthe electronic device. The electronic device is cooled by transferringheat from the electronic device to the cooling air of the heat sink.

[0023] Thus it is an object of the present invention to provide animproved heat sink design for thermal management of electronic devices.

[0024] Still another object of the present invention is the provision ofa heat sink having a cooling channel for directing cooling air to alocation adjacent an electronic device to be cooled.

[0025] Still another object of the present invention is the provision ofa heat sink which can provide additional cooling to selected locationswithout the use of heat pipes.

[0026] Still another object of the present invention is the provision ofa heat sink design of economical construction which can be engineeredfor improved cooling at selected locations.

[0027] Other and further objects, features, and advantages of thepresent invention will be readily apparent to those skilled in the art,upon a reading of the following disclosure when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a schematic plan view of a heat sink constructed inaccordance with the present invention.

[0029]FIG. 2 is a side elevation view of the heat sink of FIG. 1 showingan electronic device mounted on the base of the heat sink.

[0030]FIG. 3 is a bottom view of the heat sink of FIG. 1 showing theelectronic device mounted on the base.

[0031]FIG. 4 is an elevation section view taken along line 4-4 of FIG.1.

[0032]FIG. 4A is a view similar to that of FIG. 4 showing an alternativeembodiment of the invention wherein the cooling channel is defined by anarea of relatively short fins.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] As noted, one material from which the heat sinks of the presentinvention may be constructed is graphite sheet material. Beforedescribing the construction of the heat sinks, a brief description ofgraphite and its formation into flexible sheets is in order.

[0034] Preparation of Flexible Graphite Sheet

[0035] Graphite is a crystalline form of carbon comprising atomscovalently bonded in flat layered planes with weaker bonds between theplanes. By treating particles of graphite, such as natural graphiteflake, with an intercalant of, e.g. a solution of sulfuric and nitricacid, the crystal structure of the graphite reacts to form a compound ofgraphite and the intercalant. The treated particles of graphite arehereafter referred to as “particles of intercalated graphite.” Uponexposure to high temperature, the intercalant within the graphitedecomposes and volatilizes, causing the particles of intercalatedgraphite to expand in dimension as much as about 80 or more times itsoriginal volume in an accordion-like fashion in the “c” direction, i.e.in the direction perpendicular to the crystalline planes of thegraphite. The exfoliated graphite particles are vermiform in appearance,and are therefore commonly referred to as worms. The worms may becompressed together into flexible sheets that, unlike the originalgraphite flakes, can be formed and cut into various shapes.

[0036] Graphite starting materials suitable for use in the presentinvention include highly graphitic carbonaceous materials capable ofintercalating organic and inorganic acids as well as halogens and thenexpanding when exposed to heat. These highly graphitic carbonaceousmaterials most preferably have a degree of graphitization of about 1.0.As used in this disclosure, the term “degree of graphitization” refersto the value g according to the formula:$g = \frac{3.45 - {d(002)}}{0.095}$

[0037] where d(002) is the spacing between the graphitic layers of thecarbons in the crystal structure measured in Angstrom units. The spacingd between graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous materials include natural graphites from various sources,as well as other carbonaceous materials such as graphite prepared bychemical vapor deposition, high temperature pyrolysis of polymers, orcrystallization from molten metal solutions and the like. Naturalgraphite is most preferred.

[0038] The graphite starting materials used in the present invention maycontain non-graphite components so long as the crystal structure of thestarting materials maintains the required degree of graphitization andthey are capable of exfoliation. Generally, any carbon-containingmaterial, the crystal structure of which possesses the required degreeof graphitization and which can be exfoliated, is suitable for use withthe present invention. Such graphite preferably has a purity of at leastabout eighty weight percent. More preferably, the graphite employed forthe present invention will have a purity of at least about 94%. In themost preferred embodiment, the graphite employed will have a purity ofat least about 98%.

[0039] A common method for manufacturing graphite sheet is described byShane et al. in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In the typical practice of the Shaneet al. method, natural graphite flakes are intercalated by dispersingthe flakes in a solution containing e.g., a mixture of nitric andsulfuric acid, advantageously at a level of about 20 to about 300 partsby weight of intercalant solution per 100 parts by weight of graphiteflakes (pph). The intercalation solution contains oxidizing and otherintercalating agents known in the art. Examples include those containingoxidizing agents and oxidizing mixtures, such as solutions containingnitric acid, potassium chlorate, chromic acid, potassium permanganate,potassium chromate, potassium dichromate, perchloric acid, and the like,or mixtures, such as for example, concentrated nitric acid and chlorate,chromic acid and phosphoric acid, sulfuric acid and nitric acid, ormixtures of a strong organic acid, e.g. trifluoroacetic acid, and astrong oxidizing agent soluble in the organic acid. Alternatively, anelectric potential can be used to bring about oxidation of the graphite.Chemical species that can be introduced into the graphite crystal usingelectrolytic oxidation include sulfuric acid as well as other acids.

[0040] In a preferred embodiment, the intercalating agent is a solutionof a mixture of sulfuric acid, or sulfuric acid and phosphoric acid, andan oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solution maycontain metal halides such as ferric chloride, and ferric chloride mixedwith sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

[0041] The quantity of intercalation solution may range from about 20 toabout 350 pph and more typically about 40 to about 160 pph. After theflakes are intercalated, any excess solution is drained from the flakesand the flakes are water-washed. Alternatively, the quantity of theintercalation solution may be limited to between about 10 and about 40pph, which permits the washing step to be eliminated as taught anddescribed in U.S. Pat. No. 4,895,713, the disclosure of which is alsoherein incorporated by reference.

[0042] The particles of graphite flake treated with intercalationsolution can optionally be contacted, e.g. by blending, with a reducingorganic agent selected from alcohols, sugars, aldehydes and esters whichare reactive with the surface film of oxidizing intercalating solutionat temperatures in the range of 25° C. and 125° C. Suitable specificorganic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol,decylalcohol, 1, 10 decanediol, decylaldehyde, 1-propanol, 1,3propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose,lactose, sucrose, potato starch, ethylene glycol monostearate,diethylene glycol dibenzoate, propylene glycol monostearate, glycerolmonostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethylformate, ascorbic acid and lignin-derived compounds, such as sodiumlignosulfate. The amount of organic reducing agent is suitably fromabout 0.5 to 4% by weight of the particles of graphite flake.

[0043] The use of an expansion aid applied prior to, during orimmediately after intercalation can also provide improvements. Amongthese improvements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids which have at least 1 carbon atom, and preferablyup to about 15 carbon atoms, which is soluble in the intercalationsolution in amounts effective to provide a measurable improvement of oneor more aspects of exfoliation. Suitable organic solvents can beemployed to improve solubility of an organic expansion aid in theintercalation solution.

[0044] Representative examples of saturated aliphatic carboxylic acidsare acids such as those of the formula H(CH₂)_(n)COOH wherein n is anumber of from 0 to about 5, including formic, acetic, propionic,butyric, pentanoic, hexanoic, and the like. In place of the carboxylicacids, the anhydrides or reactive carboxylic acid derivatives such asalkyl esters can also be employed. Representative of alkyl esters aremethyl formate and ethyl formate. Sulfuric acid, nitric acid and otherknown aqueous intercalants have the ability to decompose formic acid,ultimately to water and carbon dioxide. Because of this, formic acid andother sensitive expansion aids are advantageously contacted with thegraphite flake prior to immersion of the flake in aqueous intercalant.Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids iscitric acid.

[0045] The intercalation solution will be aqueous and will preferablycontain an amount of expansion aid of from about 1 to 10%, the amountbeing effective to enhance exfoliation. In the embodiment wherein theexpansion aid is contacted with the graphite flake prior to or afterimmersing in the aqueous intercalation solution, the expansion aid canbe admixed with the graphite by suitable means, such as a V-blender,typically in an amount of from about 0.2% to about 10% by weight of thegraphite flake.

[0046] After intercalating the graphite flake, and following theblending of the intercalant coated intercalated graphite flake with theorganic reducing agent, the blend is exposed to temperatures in therange of 25° to 125° C. to promote reaction of the reducing agent andintercalant coating. The heating period is up to about 20 hours, withshorter heating periods, e.g., at least about 10 minutes, for highertemperatures in the above-noted range. Times of one half hour or less,e.g., on the order of 10 to 25 minutes, can be employed at the highertemperatures.

[0047] The thus treated particles of graphite are sometimes referred toas “particles of intercalated graphite.” Upon exposure to hightemperature, e.g. temperatures of at least about 160° C. and especiallyabout 700° C. to 1000° C. and higher, the particles of intercalatedgraphite expand as much as about 80 to 1000 or more times their originalvolume in an accordion-like fashion in the c-direction, i.e. in thedirection perpendicular to the crystalline planes of the constituentgraphite particles. The expanded, i.e. exfoliated, graphite particlesare vermiform in appearance, and are therefore commonly referred to asworms. The worms may be compressed together into flexible sheets that,unlike the original graphite flakes, can be formed and cut into variousshapes.

[0048] Flexible graphite sheet and foil are coherent, with good handlingstrength, and are suitably compressed, e.g. by roll pressing, to athickness of about 0.075 mm to 3.75 mm and a typical density of about0.1 to 1.5 grams per cubic centimeter (g/cm³). From about 1.5-30% byweight of ceramic additives can be blended with the intercalatedgraphite flakes as described in U.S. Pat. No. 5,902,762 (which isincorporated herein by reference) to provide enhanced resin impregnationin the final flexible graphite product. The additives include ceramicfiber particles having a length of about 0.15 to 1.5 millimeters. Thewidth of the particles is suitably from about 0.04 to 0.004 mm. Theceramic fiber particles are non-reactive and non-adhering to graphiteand are stable at temperatures up to about 1100° C., preferably about1400° C. or higher. Suitable ceramic fiber particles are formed ofmacerated quartz glass fibers, carbon and graphite fibers, zirconia,boron nitride, silicon carbide and magnesia fibers, naturally occurringmineral fibers such as calcium metasilicate fibers, calcium aluminumsilicate fibers, aluminum oxide fibers and the like.

[0049] The flexible graphite sheet can also, at times, be advantageouslytreated with resin and the absorbed resin, after curing, enhances themoisture resistance and handling strength, i.e. stiffness, of theflexible graphite sheet as well as “fixing” the morphology of the sheet.Suitable resin content is preferably less than about 60% by weight, morepreferably less than about 35% by weight, and most preferably from about4% to about 15% by weight. Resins found especially useful in thepractice of the present invention include acrylic-, epoxy- andphenolic-based resin systems, or mixtures thereof. Suitable epoxy resinsystems include those based on diglycidyl ether or bisphenol A (DGEBA)and other multifunctional resin systems; phenolic resins that can beemployed include resole and novolak phenolics.

[0050] When using graphite material for the heat sink, the transfer ofheat from the heat source into the base of the heat sink may be enhancedby providing a high thermal conductivity insert in the base in a mannerlike that described in U.S. patent application Ser. No. ______, entitledHEAT DISSIPATING COMPONENT USING HIGH CONDUCTIVITY INSERTS, filed Dec.13, 2001 by Krassowski, et al, the details of which are incorporatedherein by reference. A cavity is formed through the thickness of thebase and the high conductivity insert is received in the cavity. Theinsert may be an isotropic high thermal conductivity material such ascopper or an anisotropic material such as graphite oriented to have highconductivity in the direction of the thickness of the base.

THE DETAILED EMBODIMENT OF FIGS. 1-4

[0051] Referring now to the drawings, and particularly to FIG. 1, a heatsink apparatus is shown and generally designated by the numeral 10. Theapparatus 10 includes a base 12 having first and second ends 14 and 16defining a length 18 therebetween. The base 12 has a width 20 less thanthe length 18. The base 12 has a location generally designated by dashedlines 22 in FIGS. 1 and 3, which location is defined for engagement withan electronic device 24 which is to be cooled by the heat sink 10.

[0052] The electronic device 24 may be any of the devices describedabove, and will be mounted in thermal transfer operative engagement withthe base 12. The heat sink 24 may be attached to the base 12 in anyconventional manner, which may include the use of a thin thermalinterface 25 or a layer of phase change material or thermal greasetherebetween. The thermal interface 25 may be, for example, a thin layerof flexible graphite material.

[0053] A plurality of parallel fins 26 extend upward from the base 12.The fins 26 in the embodiment shown in FIG. 1 are planar fins whichextend parallel to the length 18 of the base 12. Fins 26 may bedescribed as being in two groups 28 and 30. Each of the groups 28 and 30may be described as an outer group of spaced fins having a spacing suchas 32 defined between adjacent fins. The spacing 32 will typically be auniform spacing between each of the fins, but it need not be a uniformspacing for purposes of the present invention.

[0054] The two groups of fins 28 and 30 are separated by a coolingchannel 34 extending from the first end 14 toward the location 22. Thecooling channel 34 has a channel width 36 which is substantially greaterthan the spacings 32 between adjacent fins within each of the two groups28 and 30.

[0055] A fan 42 blows air in direction 43 across the length of heat sink10.

[0056] The heat sink apparatus 10 is preferably of the type used inforced air convection, and typically the fins 26 have a thickness on theorder of from 0.5 to 1.0 mm, and the spacing 32 between adjacent fins istypically on the order of from about 1 mm to about 4 mm. The spacing istypically three times the fin thickness. In general spacing 32 willusually be less than 5 mm. In a heat sink 10 having fins 26 and spacings32 on the order just described, the channel width 36 is preferably atleast 7 mm, it is more preferably at least 10 mm and even morepreferably is about 15 mm. Another way of defining the preferred channelwidth is that the channel width is at least two times the spacing 32between adjacent fins, and more preferably at least three to four timesthe spacing between adjacent fins of the groups on either side of thecooling channel.

[0057] The cooling channel 34 has a length 38. In general when using thedesign precepts of the present invention it is desired to extend thelength 38 of cooling channel 34 to a point at or near the location 22directly above the electronic device 24 which is to be cooled. This canbe described as terminating the channel 34 at or upstream of thelocation 22. It is desired that a group of intermediate fins 40 locatedabove the location 22 receive the cooling air from the cooling channel34. Thus, one manner of describing the channel length 38 is that thechannel length should extend a substantial portion of a distance fromthe first end 14, where the fan 42 is located, to the location 22. Thelength 38 should be at least one half the distance from end 14 tolocation 22, more preferably at least 75% of the distance from end 14 tolocation 22, and most preferably the length 38 should extend from theend 14 to the location 22.

[0058] Another manner of defining the length 38 of channel 34 is withreference to the length 18 of the base 12, and in such a description,the channel 34 can be described as having a channel length 38 at leastone half the length 18 of the base 12.

[0059] It will also be understood that the definition of the location 22includes the area immediately above the electronic device 24, and asindicated best in FIG. 3, the location 22 may extend somewhat beyond theperimeter of the electronic device 24.

[0060] As best seen in FIG. 4, the cooling channel 34 may be defined byan area on the base 12 which is completely free of fins. Alternatively,as shown in FIG. 4A, the cooling channel 34 may be defined by an area ofrelatively short fins 26A.

[0061] As seen in FIGS. 4 and 4A, the fins 26 have a fin height 27 abovethe base 12, which fin height 27 is typically on the order of 20 to 50mm.

[0062] In one embodiment, the innermost fins 26B immediately on eitherside of cooling channel 34 may simply be cooling fins constructed in thesame manner and of the same material as the other cooling fins 26.Alternatively, however, the innermost fins 26B may be constructed of aninsulating material and serve as insulating walls 26B defining the sidesof the cooling channel 34.

[0063] As previously mentioned, the heat sink 10 including its base 12and fins 26 may be made of any conventional heat sink materialsincluding copper, aluminum, graphite, and composites of the above.

[0064] The cooling channel 34 of the present invention has been found tobe particularly effective in relatively elongated heat sinks. Theelectronic device 24 is located away from the end 14 which is adjacentthe source of cooling air such as fan 42. Such an elongated heat sinkmay be characterized by a length 18 at least three times as great as thewidth 20.

[0065] With such an elongated heat sink, if the heat sink were ofconventional design having fins across the entire width and the entirelength, the air passing from fan 42 to the location 22 immediately aboveelectronic device 24 could be so heated by heat from the fins thatinsufficient cooling is provided in the location 22. It has beendetermined that by essentially eliminating certain fins so as to definethe cooling channel 34, cooling air from fan 42 will pass directlythrough the channel 34 with relatively little heating occurring to theair, and thus cool air passes through the fins 40 directly above thelocation 22. In certain situations, the loss of cooling efficiency bythe elimination of fins from the area of cooling channel 34 is more thanoffset by the increased cooling efficiency due to cooler air passingover the location 22. Reduced operating temperatures of the electronicdevice 24 on the order of 1.5° to 3° C. may be achieved, which as willbe appreciated by those skilled in the art may be very significant.

[0066] The following example shows one situation where the coolingchannel of the present invention can be utilized effectively.

[0067] A heat sink design like that shown in FIGS. 1-4 was modeled usingcomputational fluid dynamic modeling.

[0068] Case No. 1 was for an elongated copper heat sink with no coolingchannel. The heat sink was modeled having a length 18 of 280 mm, a width20 of 70 mm, a base thickness of 8 mm, a total of 27 fins having finthickness of 0.635 mm, a spacing between fins of 2.0 mm, and fin heightof 37 mm. The heat source was modeled as having a size of 40 mm by 40 mmand having a power output of 150 W. The copper material for the heatsink had a thermal conductivity of 391 W/m ° C.

[0069] In Case No. 2, the design is similar to No. 1 except the ninecentralmost fins have had a portion thereof removed along the length 38as seen in FIG. 1, which length is 168 mm. The channel width 36 is equalto 26 mm.

[0070] Case No. 3 is similar to Case No. 2 except the model utilizedgraphite material having a thermal conductivity of 400 W/m ° C. in theplane of the base and utilizing a copper insert in the base directlyabove the heat source, like that described above with reference to U.S.patent application Ser. No. ______, entitled HEAT DISSIPATING COMPONENTUSING HIGH CONDUCTIVITY INSERTS.

[0071] Case No. 4 utilized the same materials as Case No. 3, but added acooling channel having dimensions like that of Case No. 2.

[0072] Case No. 5 is similar to Case No. 1 except that the material ofthe heat sink modeled was aluminum having a thermal conductivity of 209W/m ° C.

[0073] Case 6 utilized the same aluminum material as Case No. 5, butadded a cooling channel having dimensions like that described above forCase No. 2.

[0074] Utilizing the same ambient conditions for each of the six cases,the maximum temperature of the base 12 in the location 22 (T_(max)) andthe thermal resistance (R_(sa)) were calculated and are shown in thefollowing Table I. TABLE I Case Heat Sink Model T_(max) R_(sa) No.Material Option (° C.) (° C./W) 1 Copper Original 60.44 0.24 Design 2Cooling 58.43 0.23 Channel 3 Graphite Original 62.27 0.25 Design 4Cooling 60.48 0.24 Channel 5 Aluminum Original 68.04 0.29 Design 6Cooling 65.25 0.27 Channel

[0075] As seen in Table I, when comparing Cases 2 and 1, the addition ofthe cooling channel to the copper heat sink lowered T_(max) from 60.44°C. to 58.43° C., thus providing a temperature decrease of 2.01° C.

[0076] Looking now at the graphite material example in Cases 3 and 4,the addition of the cooling channel in Case 4, as compared to the designwithout the cooling channel in Case 3, resulted in a decrease in T_(max)from 62.27° C. to 60.48° C. or an improvement of 1.79° C.

[0077] Finally, comparing Cases 6 and 5, the addition of the coolingchannel to the aluminum heat sink resulted in the most significantdecrease, from 68.04° C. to 65.25° C. for an improvement of 2.79° C.

[0078] Thus it is seen that in a heat sink having dimensions and ageometry like that described in the example above, the addition of acooling channel upstream of the location of the heat source providedsignificant decreases in the maximum temperature of the heat sink at theheat source, which are all within a range of from about 1.5° C. to about3.0° C. for the example model.

[0079] Also, the elimination of fins in the area of the cooling channelreduces the weight of the heat sink by as much as about 15%. This is anadvantage in many products.

[0080] Methods of utilizing the heat sink of the present inventioninclude:

[0081] (a) providing the heat sink 10 having first and second groups 28and 30 of fins 26 and having the cooling channel 34 defined between thefirst and second groups of fins 28 and 30;

[0082] (b) placing the electronic device 24 in heat transfercommunication with the location 22 on the heat sink 10;

[0083] (c) channeling cooling air from the fan 42 through the coolingchannel 34 to the location 22 of the electronic device 24; and

[0084] (d) cooling the electronic device 24 by transferring heat fromthe electronic device 24 to the cooling air via the heat sink 10.

[0085] Thus it is seen that the apparatus and methods of the presentinvention readily achieve the ends and advantages mentioned as well asthose inherent therein. While certain preferred embodiments of theinvention have been illustrated and described for purposes of thepresent disclosure, numerous changes in the arrangement and constructionof parts and steps may be made by those skilled in the art, whichchanges are encompassed within the scope and spirit of the presentinvention as defined by the appended claims.

What is claimed is:
 1. A heat sink apparatus, comprising: a base havingfirst and second ends; a plurality of fins extending upward from thebase; and a cooling channel defined between first and second groups ofthe fins, the cooling channel extending from the first end toward alocation for an electronic device, the cooling channel having a channelwidth greater than a spacing between adjacent fins within each of thefirst and second groups, the cooling channel terminating upstream of thelocation for the electronic device.
 2. The apparatus of claim 1,wherein: the plurality of fins includes intermediate fins located abovethe location for the electronic device in the path of the coolingchannel, so that cooling air is directed by the cooling channel alongthe intermediate fins.
 3. The apparatus of claim 1, wherein: the firstand second ends of the base define a length of the base therebetween,the base having a width less than the length; and the fins extendparallel to the length of the base.
 4. The apparatus of claim 1, whereinthe channel width is at least 7 mm.
 5. The apparatus of claim 1, whereinthe channel width is at least 10 mm.
 6. The apparatus of claim 1,wherein the channel width is at least three times the greatest spacingbetween adjacent fins within each of the groups of fins.
 7. Theapparatus of claim 6, wherein the cooling channel has a channel lengthat least one-half a distance from said first end to said location. 8.The apparatus of claim 1, wherein the cooling channel has a channellength at least one-half a distance from said first end to saidlocation.
 9. The apparatus of claim 8, wherein the channel length is atleast 75% of the distance from said first end to said location.
 10. Theapparatus of claim 1, wherein the cooling channel has a channel lengthat least one-half the length of the base between the first and secondends.
 11. The apparatus of claim 1, wherein the cooling channel isdefined by an area on the base which area has no fins.
 12. The apparatusof claim 1, wherein the cooling channel is defined by an area on thebase having fins of shorter height than the fins of the groups of fins.13. The apparatus of claim 1, wherein design parameters of the apparatusare such that a loss in cooling efficiency resulting from the presenceof the cooling channel, as contrasted to having additional fins in thearea of the cooling channel like the fins of the groups, is more thanoffset by an increase in cooling efficiency due to cooler air beingchanneled to the location of the electronic device.
 14. The apparatus ofclaim 1, wherein the heat sink is at least partially constructed from agraphite material
 15. The apparatus of claim 1, further comprisinginsulating walls defining sides of the cooling channel.
 16. Theapparatus of claim 1, in combination with: a cooling fan oriented todirect cooling air across the heat sink from the first end toward thesecond end of the base; and an electronic device in heat transfercommunication with the location on the base, so that heat from theelectronic device is transferred by the heat sink from the electronicdevice to the cooling air.
 17. A heat sink apparatus, comprising: a basehaving a length and a width, the length being at least three times thewidth; and a plurality of parallel fins extending from the base parallelto the length of the base, the fins including first and second groupsseparated by a cooling channel extending from one end of the base atleast one-half the length of the base and less than the entire length ofthe base.
 18. The apparatus of claim 17, wherein: the plurality of finsincludes intermediate fins located in the path of the cooling channelabove a location for an electronic device, so that cooling air isdirected by the cooling channel to the intermediate fins.
 19. Theapparatus of claim 17, wherein the cooling channel has a channel widthof at least 7 mm.
 20. The apparatus of claim 17, wherein adjacent finsof each group are spaced apart by less than 5 mm.
 21. The apparatus ofclaim 17, wherein the cooling channel is defined by an area on the basewhich area has no fins.
 22. The apparatus of claim 17, wherein thecooling channel is defined by an area on the base having fins of shorterheight than the fins of the groups of fins.
 23. A heat sink apparatus,comprising: a base having a length; and a plurality of parallel finsextending from the base, the plurality of parallel fins including firstand second outer groups of fins extending the entire length of the base,and a third intermediate group of fins extending less than the entirelength of the base, so that a cooling channel is defined between thefirst and second outer groups of fins in an area of the base not coveredby the third intermediate group of fins.
 24. The apparatus of claim 23,wherein the plurality of parallel fins has an equal spacing within eachof the first, second and third groups.
 25. The apparatus of claim 23,wherein the cooling channel has a channel length at least one half thelength of the base.
 26. The apparatus of claim 23, wherein designparameters of the apparatus are such that a loss in cooling efficiencyresulting from the presence of the cooling channel, as contrasted tohaving additional fins in the area of the cooling channel like the finsof the groups, is more than offset by an increase in cooling efficiencydue to cooler air being channeled to the location of the electronicdevice.
 27. A method of cooling an electronic device, comprising:providing a heat sink having first and second groups of fins and havinga cooling channel defined between the first and second groups; placingthe electronic device in heat transfer communication with a location onthe heat sink; channeling cooling air through the cooling channel to thelocation of the electronic device; and cooling the electronic device bytransferring heat from the electronic device to the cooling air via theheat sink.
 28. The method of claim 27, further comprising: providingmore densely packed fins on the heat sink at the location of theelectronic device than are provided upstream of the location.